New idea for multiple flying extruders: Double-Decker Delta

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626Pilot
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New idea for multiple flying extruders: Double-Decker Delta

Post by 626Pilot »

I've been working on the problem of good retracts and print quality for awhile, as have many others. The best way I know to do this is to shorten the Bowden tube as much as possible, and that goes double for anyone who wants to mess with flexible filaments such as TPU.

One of the earliest solutions I saw was the Air Struder by Jassper. Later, Trick Laser debuted the Fly-N-Strude. These are both adequate for running a single extruder. I wanted three (for a Diamond hot end) or three plus one up top for support material (for a Kraken), so earlier this year I designed an arm-mounting system for Trick Laser arms:
EZStruder TL Arm Mount Detail.jpg
This worked fairly well, but it didn't let the extruder bracket swivel, so it was always pointed in the same direction. As a result, it worked very well for small prints, but the tube would have to bend a lot on larger prints. That added friction - sometimes a lot of friction! After using it for a few months, I decided to design a triple-extruder flying platform:
Cable mast.jpg
This works a lot better, but is still flawed. One thing it does much better, but not nearly ideal, is lower bowden tube flexion than the arm mounts. The more bent the tube is, the more friction it adds to the filament path, wrecking retraction performance and making it that much harder for the extruder to push the right amount of filament at the right time. When the effector is dead center, there's no flexion (or very little); but the further we move from the center in the XY plane, the more the platform wants to "fall over." Another issue: the extrusion steppers will tend to "stand on" the hot end's effector platform, transferring their weight down to it through both the filament and the Bowden tube. As with flexion, the weight exerted varies with the XY position of the effector. I have the paracord and bowden tube lengths balanced so that it's just perfect when centered, but as soon as it moves off center, it will exhibit this pushing effect.

In practice, this doesn't hurt accuracy that much - on brand-new 300mm Trick Laser arms. I can get a nice calibration on my printer with an average Z error of ~20 microns, which I think is excellent. However, the differential loading is likely to make the joints wear out faster. I think we all prefer to have printers where the arms aren't considered a wear item!

One solution would be to use pairs of paracord "arms" rather than a single "arm" that lets it flop around so much, but the kinematics still won't keep the extruder platform centered perfectly above the effector. If the paracord comes from the same elevation as the delta arms, the cord must necessarily be shorter than the arms in order to maintain the extruder platform above the effector. That changes the equation of where the platform ends up: it never moves away from center as fast as the effector because its "arms" don't allow it to go as far. If the effector is 100mm out, the extruder platform is perhaps 85mm out, for example. It can't help but lean over in that configuration.

Then, last night, it hit me: Take the existing carriages, arms, and effector, and make a "copy" of them that would go higher up on the belts. To visualize this, look at the above picture and imagine a second set of carriages a few inches above the first, and delta arms projecting from the upper carriages to the triple-extruder platform. The platform would be modified such that it would have joints at the same position as the effector, on the curved white crossmembers that connect the extruder brackets. Spacing between the upper and lower carriages would be achieved with a printed part that would be fastened to both of them.

Advantages:
  • You'd get the same performance as you would with a direct-drive extruder, or very close to it. Some people have abandoned the delta platform altogether over this issue.
  • Same-length arms and identical joint positions on extruder and effector platforms make the kinematics for both identical. The extruder platform is GUARANTEED to be exactly above the effector.
  • No bending of the Bowden tubes.
  • No differential loading exerted on the effector by the extruder platform.
  • Requires relatively little PTFE tubing.
  • No careful tuning of the length of the paracord is required.
  • Drivetrain exerts positive control over the massive stepper platform at all times. It will not bounce around, or risk coming to rest on one of the arms when the effector is close to the print radius.
  • The belts are under constant tension, which (maybe?) helps with accuracy.
  • It looks freaking cool.
Disadvantages:
  • It costs more money to do it this way.
  • You lose a few inches of print height (~3-4?) because the hot end carriages have to be lower on the belts than they were before, although this could be overcome with a sufficiently clever design (see below).
  • As currently designed, the extruder platform is wide enough that it could interfere with the lower (effector) arms when the effector is close to one of the towers. It seems possible to redesign it to be more "concave," but you still might lose an inch or so of print radius.
  • No room for a fourth extruder. One could be mounted up top, or somewhere else. It wouldn't get the benefit of the super-short Bowden tubes. I figure you'd probably use it for support material, where perfect feed is less important.
  • Requires a counterweight system so that the hot end doesn't dive into the glass at 15MPH when you turn the printer off. (This is not a hard problem - I'm working on it now.)
Solutions to the lost print height issue:
  • Heavy-duty rod ends, something that could actually take 3-4lbs. of weight (divided by six of course), could be used, and the extruder platform could sit on top of a short tower that would bolt to the extruder platform. Three fewer carriages, six fewer arms, and no height lost, although it would make it significantly harder to keep the effector platform accurate. Tensioned ball arms (chromed steel on either end of the rods, riding in acetal cups on the effector and carriages, tensioned by paracord or similar) might be good enough. Another solution would be to use tiny NEMA 11 geared steppers, as suggested by bot. They wouldn't add as much weight. Unfortunately, the only ones I know of are sold by Warner Berry, and fully 3x as expensive as the TriDPrinting geared NEMA 17 solution. Then again, we would be saving money from not having to buy another six arms.
  • Alternately, keep the two-sets-of-arms-solution, but push the carriage joints inwards 2" towards print center. In this way, they would totally clear the endstop switches. The lower (effector) joints would be at the same elevation as now, and the upper (extruder platform) joints would be mounted on a mast that projects upwards. The carriages and their masts would have to be engineered to withstand the cantilevered supports projecting upwards several inches. When the printer is homed, the upper joints would be several inches above the endstops!
What do you think? Do you have a different solution, or some ideas for how to make this one better?
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Re: New idea for multiple flying extruders: Double-Decker De

Post by ccavanaugh »

How about a 4th asymmetrical tower with the same belt and carriage setup with the extruder directly coupled to it?

Z height would be calculated as a function of print z height plus a bit of extra wiggle room. It was take a little bit of firmware modification, but the carriage could easily handle the load of three extruders.

This allows use of stock arms, ball ends, etc and maintains positioning accuracy. Also, it should not impact available print height.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by 626Pilot »

Initially that sounded like a really good idea, but then I realized that there's nowhere you could mount the 4th tower that wouldn't interfere with the delta arms. No matter where you put the tower along the circumference, there will be some place the effector can no longer go.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by ccavanaugh »

626Pilot wrote:Initially that sounded like a really good idea, but then I realized that there's nowhere you could mount the 4th tower that wouldn't interfere with the delta arms. No matter where you put the tower along the circumference, there will be some place the effector can no longer go.
It would have to be pushed outside the normal envelope diameter and you would have a longer bowden tube than you would if using the flying struder approach. Just a wag, but I would think the bowden length would be about a 1/2 of what it is today which may not be enough gain.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by Mac The Knife »

Still on my first cup of coffee,,,, but why not just move the two effector arms to the bottom of the carriage, and at the top of the carriage, mount one arm per carriage for the extruder platform? by itself, the extruder platform would need two arms per carriage, but the bowden tubes to the hotend should keep it stable.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by mhackney »

The 4th tower idea might have a different take - what if the tower is movable, mounted from the top at the center? The extruders could all be mounted on it - could probably get 3 at least around it. It might have to elevate them above the top most point of the carriages but that could be easily arranged. This 4th tower would then move up and down in concert with the Z level of the platform.

This would remove all extruder mass from the delta platform and allow shortening the Bowden tubes considerably.

Also, have you tried the 1.8mm ID PTFE tubing? I'm getting much better retraction results with it.

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Re: New idea for multiple flying extruders: Double-Decker De

Post by ccavanaugh »

mhackney wrote:The 4th tower idea might have a different take - what if the tower is movable, mounted from the top at the center? The extruders could all be mounted on it - could probably get 3 at least around it. It might have to elevate them above the top most point of the carriages but that could be easily arranged. This 4th tower would then move up and down in concert with the Z level of the platform.

This would remove all extruder mass from the delta platform and allow shortening the Bowden tubes considerably.

Also, have you tried the 1.8mm ID PTFE tubing? I'm getting much better retraction results with it.
Good point, the central location would provide better clearance. The smaller 20mm extrusion could be used to save a little bit of space. Also, take a look at http://hackaday.com/2015/12/24/3d-print ... onversion/. A variation of this could eliminate the need for the extrusion completly. Positional accuracy is not needed and a printed rack and pinion would work well with a smaller Nema 11 motor.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by mhackney »

Yes, I should have mentioned the rack and pinion, thats what I was thinking too. Even commercial R&P are cheap.

On thinking further about this, I think it has legs!

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Re: New idea for multiple flying extruders: Double-Decker De

Post by mhackney »

Extruder Post.jpg
Sorry for the poor quality but this is a quick look at the "extruder post" idea. I think as long as the extruders themselves are just a bit above the top of the carriages, they won't interfere at all. This still significantly shortens the Bowden path and more importantly, straightens it too. With 1.8mm PTFE there would be even less slop.

The mechanics with a rack and pinion would be easy. The electronics to follow the effector don't need to be over complex either. A mechanical contact post from the effector up to the bottom of the tower could act as a switch - when the contact is broken, it moves the tower down. It could even be one of the distance measuring technologies - IR, ultrasonic, etc. But a mechanical arrangement would be quick to do a proof of concept.

The Metal Max would be a great platform to test this with.

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Re: New idea for multiple flying extruders: Double-Decker De

Post by bot »

This all goes back to that idea of a counter-weight. I believe werner berry does this. Replace that center column with a rope and pulley with a counter-weight on the other end, to allow the extruders to float. No added electronics, very simple and cheap mechanics.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by 626Pilot »

I am designing a counterweight system. Each stepper/EZStruder combo weighs slightly over a pound, which is close to 500g, so I got three 500g weights and some V-groove pulleys. I'll tie fishing line to the carriages and then use a combination of pulleys and printed parts to route the lines to the back of the printer, behind the Z tower, where they will be allowed to hang. I decided against using a suspension system like Berry's because it doesn't pull upwards from all angles, and I didn't want something side-loading the extruders when the effector is away from bed center. If we take the weight off at the carriages instead, it's always transferred along a perfect up-vector.

The overhead pillar idea is similar to Berry's, but more parts are involved and it doesn't seem like it would be able to pivot and let the platform stay above the effector. It would definitely require a stepper and a rack-and-pinion system, and logic to maintain a constant distance between the platform and the effector. That means that however high up it is when the effector is centered, it will have to go lower as the effector moves away from center. If it doesn't, the Bowden tubes will eventually run out of slack and the two platforms will end up pulling against each other. This happened with my arm-mounted solution when the effector was way out close to the edge. The friction got so bad that it would make the extruder skip! I would also prefer to avoid a solution that involves adding another stepper + logic in the firmware. I want the solution to be completely mechanical.

I'm starting to think the solution of a single set of tensioned ball arms in acetal cups, and the extruder platform on a tower bolted to the effector, might be the simplest way to do this. It would add a lot of inertia, and that inertia would have significant leverage to act against the effector. Something like this 27:1-geared NEMA 11 stepper might be the answer. It weighs about 1/4 as much as the geared NEMA 17 motors I have now. They're more expensive, at $54 each, but arms are also expensive. This might completely eliminate the need for a counterweight system, although I might still use one so the effector doesn't want to descend when the power is turned off.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by Glacian22 »

Here's the motor that I'm going with for a similar setup: http://www.omc-stepperonline.com/gear-r ... p-263.html" onclick="window.open(this.href);return false;

It's also a 27:1 geared Nema 11, 120 grams, and only $32. Caveats: it has an exposed rear shaft, and the main drive shaft appears to be 6mm, not many hobbed gears have that size bore. I'm going to try printing a ring to use as a spacer to use with an 8mm bore hobbed gear, and if that doesn't work I'll try drilling out a 5mm one. :)
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Re: New idea for multiple flying extruders: Double-Decker De

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Glacian22 wrote:Here's the motor that I'm going with for a similar setup: http://www.omc-stepperonline.com/gear-r ... p-263.html" onclick="window.open(this.href);return false;

It's also a 27:1 geared Nema 11, 120 grams, and only $32. Caveats: it has an exposed rear shaft, and the main drive shaft appears to be 6mm, not many hobbed gears have that size bore. I'm going to try printing a ring to use as a spacer to use with an 8mm bore hobbed gear, and if that doesn't work I'll try drilling out a 5mm one. :)
Drill out the 5mm one, I've drills them out to 8mm before now. The existing bore should self center the drill.
Even with a lathe a spacer with thin walls like that, like that would be difficult to make concentric..
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Re: New idea for multiple flying extruders: Double-Decker De

Post by bot »

Glacian22 wrote:Here's the motor that I'm going with for a similar setup: http://www.omc-stepperonline.com/gear-r ... p-263.html" onclick="window.open(this.href);return false;

It's also a 27:1 geared Nema 11, 120 grams, and only $32. Caveats: it has an exposed rear shaft, and the main drive shaft appears to be 6mm, not many hobbed gears have that size bore. I'm going to try printing a ring to use as a spacer to use with an 8mm bore hobbed gear, and if that doesn't work I'll try drilling out a 5mm one. :)

I got a similar motor. Mine is 5:1, as even that is a lot of redutction. It's worth noting that 120g is the weight of the motor alone, without the gearbox. The whole unit will come in at about 200g. I also got the longer, more powerful gearbox with the 5:1 in case the shorty can't push the filament enough.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by 626Pilot »

I'm looking at the same problem. I got an overpriced NEMA11 geared motor with a 6mm shaft. The Tatsu v2 drive gear is available with 5mm and 6mm bores, but it's twenty-five freaking dollars. Granted I don't know much about hobbing things, but it seems like TriDPrinting sells 5mm and 8mm ID drive gears for a third of that, and they have somehow not gone out of business yet.

What's the best way to bore a 5mm drive gear out to 6mm, in a really accurate way? (Assume I don't have $1500 for a nice lathe.)
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Re: New idea for multiple flying extruders: Double-Decker De

Post by KAS »

Wonder if a 6mm reamer on a drill press would work?
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Re: New idea for multiple flying extruders: Double-Decker De

Post by Xenocrates »

It is unlikely a reamer would work. The size of the guide on the end will not fit in a 5MM hole. I would suggest trying to find a cheap dial indicator on an arm, and attempt to center it using a fixture that clamps around it, and then clamp that to the table of a drill press. Either that, or just use the angle on the drill to center it. That will likely be close enough, so long as you use a good new bit.
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Re: New idea for multiple flying extruders: Double-Decker De

Post by rubiks24 »

What I plan on doing is picking up these http://www.helidirect.com/catalog/produ ... redirect=1 Graupner Prop Reducers 8mm to 6mm and using it with my existing 8mm hobbed gear. Probably have to use a slightly longer set screw.
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